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Brooke Greenberg is the size of an infant, with the mental capacity of a toddler.
She turned 16 in January.
"Why doesn't she age?" Howard Greenberg, 52, asked of his daughter. "Is she the fountain of youth?"
Such questions are why scientists are fascinated by Brooke. Among the many documented instances of children who fail to grow or develop in some way, Brooke's case may be unique, according to her doctor, Johns Hopkins School of Medicine pediatrician Lawrence Pakula, in Baltimore.
"Many of the best-known names in medicine, in their experience ... had not seen anyone who matched up to Brooke," Pakula said. "She is always a surprise."
Watch "20/20" FRIDAY at 10 p.m. ET for Brooke's story.
Brooke hasn't aged in the conventional sense. Dr. Richard Walker of the University of South Florida College of Medicine, in Tampa, says Brooke's body is not developing as a coordinated unit, but as independent parts that are out of sync. She has never been diagnosed with any known genetic syndrome or chromosomal abnormality that would help explain why.